1. Field of the Invention
The present invention generally relates to a light source device, an optical scanning device, and an image forming apparatus, and more particularly relates to a light source device including a surface emitting laser or a surface emitting laser array and having light intensity control and polarization control mechanisms; an optical scanning device including a beam-splitting unit for splitting a laser beam and thereby enabling the detection of the intensity of the light beam; and an image forming apparatus including the light source device and/or the optical scanning device.
2. Description of the Related Art
In an image forming apparatus such as a laser printer or a digital copier, a latent image (electrostatic latent image) is formed on a photoconductor by focusing a light beam, which is emitted from a light source and modulated according to image data, via a beam deflector and a scanning lens onto the photoconductor and by scanning the focused light beam in a specified direction (main-scanning direction). The formed latent image is developed by causing toner to adhere to the latent image.
In recent years, there has been a demand for improving the printing speed and printing resolution of such an image forming apparatus. One way to improve the printing speed and resolution of an image forming apparatus is to increase the light scanning speed and resolution of an optical scanning device of the image forming apparatus by increasing the deflection speed of a beam deflector, in other words, by increasing the rotational speed of a polygon mirror of the optical scanning device.
However, a higher rotational speed increases noise and heat generated by an optical scanning device and therefore it is difficult to increase the rotational speed of a polygon mirror above a certain level. Another way to increase the light scanning speed and resolution of an optical scanning device is to scan multiple lines at once using multiple light beams.
For example, there is a multi-beam light source device that can scan multiple light beams using a multi-beam light source for generating multiple light beams. Such a multi-beam light source may be implemented by a laser array having multiple light-emitting points in one package. The printing speed and resolution of an image forming apparatus can be improved by replacing a conventional optical scanning device having a single-beam light source device with an optical scanning device having a multi-beam light source device. Also, many technologies for implementing a multi-beam light source device using multiple single-beam light sources each having one light emitting point have been proposed.
As a light source, a laser diode (LD) called an edge emitting laser had been mainly used. In recent years, however, a laser diode called a vertical cavity surface emitting laser (VCSEL, hereafter called “surface emitting laser”) has come to be used. Compared with edge emitting lasers, it is easier to form a laser array using surface emitting lasers. For example, with edge emitting lasers, an array of 4 to 8 light beams may be the maximum. On the other hand, with surface emitting lasers, it is possible to array 16 to 32 or more light beams. For this reason, surface emitting lasers are expected to improve the printing speed and resolution of image forming apparatuses. Also, surface emitting lasers may be used for other optical devices such as an optical communications system.
However, to use a surface emitting laser instead of an edge emitting laser as a light source for a conventional optical scanning device, some problems as described below must be solved.
Normally, the light output level of an edge emitting laser is automatically controlled based on feedback obtained by monitoring at least a portion of light emitted backward from the edge emitting laser. This output level control is called auto power control (APC). In the case of a surface emitting laser, since it does not emit light backward, a different light intensity control mechanism is necessary. Without light intensity control, the light output level of a light source device fluctuates. The fluctuation in the light output level causes unevenness in image density and thereby makes it difficult for an image forming apparatus using the light source device to produce a high-quality image.
One way to control the light intensity of a surface emitting laser is to split a light beam emitted from the surface emitting laser and thereby to direct a certain portion of the light beam to a photodetector. Based on an output from the photodetector, a laser beam intensity control unit controls the driving current of the surface emitting laser and thereby maintains the light output from the surface emitting laser at a certain level. In this case, a beam splitter as shown in FIG. 18 or a half mirror as shown in FIGS. 19, 20A, and 20B may be used to split a light beam and thereby to direct a portion of the light beam to a photodetector.
Also, patent documents 1 through 7 and non-patent document 1 disclose technologies for solving the above problems. Patent document 1 discloses a light intensity monitor including a beam splitter positioned adjacent to a surface emitting laser and a photodetector positioned close to the surface emitting laser (FIG. 27). In the disclosed light intensity monitor, the beam splitter separates a portion of a light beam emitted from the surface emitting laser and the photodetector receives the separated portion of the light beam. Also, the disclosed light intensity monitor is designed so as not to greatly increase the size and costs of a laser-using apparatus.
Patent documents 2 and 3 disclose technologies for separating a portion of a light beam by using a half mirror as a beam-splitting optical element. Patent document 2 discloses an optical scanning device that can make changes in reflectance and transmittance, which changes are caused by difference in angle of view, substantially the same among multiple light beams (FIG. 29).
Patent document 3 discloses an optical scanning device that prevents fluctuation in a driving current from affecting the ratio of the intensity of a laser beam transmitted through a half mirror to that of a laser beam reflected by the half mirror (FIG. 28).
However, technologies disclosed in the above patent documents have disadvantages as described below.
In the light intensity monitor disclosed in patent document 1, a portion of a laser beam emitted from a surface emitting laser is separated by a beam splitter adjacent to the surface emitting laser. The disclosed light intensity monitor can maintain the light intensity of a light beam just emitted from a surface emitting laser at a certain level. However, since the divergence angle of a light beam emitted from a surface emitting laser changes as the driving current changes, the amount of the light beam that can pass through an aperture, which is a beam-limiting unit provided between a surface emitting laser and a beam deflector, may change.
Also, since the beam splitter in the light intensity monitor disclosed in patent document 1 is positioned in the path of divergent light, the portion of the light beam separated by the beam splitter and received by the photodetector is also divergent light. Therefore, to receive all of the separated divergent light, it is necessary to increase the size of the photodetector. However, increasing the size of a photodetector may reduce the responsivity of the photodetector. Although it is possible to reduce the size of a photodetector by adding a light focusing unit, such a configuration increases production costs.
To obviate the above problems, in patent documents 2 and 3, a beam-splitting unit (half mirror) for separating a portion of a light beam is positioned downstream of an aperture. This configuration makes it possible to maintain the intensity of a light beam passing through an aperture at a certain level. With this configuration, however, the position of a photodetector becomes further from the surface emitting laser than that in the configuration shown in patent document 1.
Also, in an optical scanning device disclosed in patent document 2 or 3, a portion of a light beam is separated and directed in a direction that forms a wide angle with the light path of a light beam going from the light source to the beam deflector. Therefore, a photodetector must be placed in a position that is distant from the light source. Such a configuration contributes to increasing the size of an optical scanning device. Although it is possible to position the photodetector close to the light source by providing a loopback mirror for bending the separated light beam toward the light source, such a configuration increases production costs. Also, as in the case of patent document 1, to reduce the size of the photodetector, an additional light focusing unit is necessary.
Further, in both of the above configurations, since reflection of a beam splitter or a half mirror is used to split a light beam, the beam splitter or the half mirror must be installed with very high accuracy to guide a reflected light beam to a desired point.
FIG. 27 is a drawing illustrating a conventional optical scanning device where a beam splitter is used to separate a portion of a light beam. FIG. 60 is a drawing illustrating a conventional optical scanning device where a half mirror is used to separate a portion of a light beam.
In an optical scanning device disclosed in patent document 1, a beam splitter 341 is used to split a light beam and thereby to guide a portion of the light beam to a photodetector 340 (FIG. 27). In an optical scanning device disclosed in patent document 4, a half mirror 44 is used as a beam-splitting optical element to separate a portion of a light beam (FIG. 60).
In an optical system 343 shown in FIG. 60, reference numbers are assigned only to a light intensity control circuit 342, a half mirror 344, a polygon mirror 348, and imaging lenses 346 and 347 for scanning a light beam onto a photosensitive drum 345.
A light beam emitted from an edge emitting laser is linearly polarized in a direction parallel to the active layer of the edge emitting laser. FIG. 61 is a drawing illustrating the polarization direction of a light beam emitted from an edge emitting laser.
While a light beam emitted from an edge emitting laser is linearly polarized as shown in FIG. 61, a light beam emitted from a surface emitting laser is randomly polarized due to its structure. Therefore, a polarization control mechanism is necessary for a light beam from a surface emitting laser.
In an optical scanning device, a light beam is transmitted and reflected by many optical elements such as a beam deflector. The transmission and reflection of a light beam at a phase boundary is polarization-dependent. In other words, the transmittance and reflectance of a light beam polarized in a direction parallel to the incidence plane (P-polarized) and those of a light beam polarized in a direction perpendicular to the incidence plane (S-polarized) become different.
Therefore, light beams emitted from the light sources in a multi-beam light source device are preferably polarized in the same direction. Also, when a light source device having surface emitting lasers is used in an optical scanning device that is originally designed to be used with a light source device having edge emitting lasers, the polarization directions of the light beams are preferably the same. If the light beams have different polarization directions, the light intensity characteristics in one line (one scan line) are greatly degraded.
Accordingly, with an image forming apparatus having such characteristics, image density becomes uneven because of the polarization dependence of transmittance and reflectance of a beam deflector and other optical elements, and therefore it is difficult to form an image with high quality.
FIG. 62 is a drawing illustrating a conventional polarization control mechanism. Patent document 5 discloses a polarization control mechanism for a surface emitting laser. The disclosed polarization control mechanism is incorporated in the structure of a surface emitting laser (FIG. 62). One disadvantage of the disclosed polarization control mechanism is that the structure of a surface emitting laser becomes complicated and therefore its production is difficult.
Also, the structure of the disclosed polarization control mechanism may have to be changed according to the structure and production method of a surface emitting laser. Therefore, the disclosed polarization control mechanism may not be able to be used for all types of surface emitting lasers. As another example, there is a polarization control method where the polarization of a light beam emitted from a surface emitting laser is controlled before the light beam is affected by the polarization dependence of transmittance and reflectance of optical elements.
FIG. 63 is a drawing illustrating a conventional polarization control mechanism disclosed in patent document 6 where a polarization beam splitter is positioned adjacent to the light emitting side of a surface emitting laser. FIG. 29 is a drawing illustrating a conventional polarization control mechanism disclosed in patent document 2 where a polarizer and a half mirror are used.
The polarization control mechanism disclosed in patent document 6 is provided outside of the light emitting part of a surface emitting laser. More specifically, in patent document 6, a polarization beam splitter 350 is positioned adjacent to the light emitting side of a surface emitting laser 349 (FIG. 63). The polarization beam splitter 350 transmits only a light beam with a specific polarization direction. In the polarization control mechanism disclosed in patent document 2, a polarizer 351 and a half mirror 352 are positioned in the light path of a light beam so that only a light beam with a specific polarization direction is transmitted (FIG. 29).
However, in devices disclosed in patent documents 1 and 6, light intensity control is not performed. In an optical scanning device disclosed in patent document 2, both a polarization control unit and a beam-splitting unit are provided. However, those two units are provided separately. In an optical scanning device disclosed in patent document 7, polarization direction of a light beam is controlled by a polarization filter.
In a light intensity control unit disclosed in patent document 1, a light beam is split by a beam splitter and a cover glass (parallel plate), and a portion of the light beam is thereby directed to a photodetector. In a light intensity control unit disclosed in patent document 4, a beam-splitting optical element (half mirror) is used.
A polarization control mechanism disclosed in patent document 5 uses a resonator structure to control the polarization of a light beam. In a polarization control mechanism disclosed in patent document 6, a polarization beam splitter is positioned adjacent to the light emitting side of a surface emitting laser. In an optical scanning device disclosed in patent document 2, a polarization control unit is provided between a light source unit and a deflecting unit. Also, the polarization control unit is integrated with another optical element. Further, a light intensity detecting unit is provided between the polarization control unit and the deflecting unit.
In an optical scanning device disclosed in patent document 7, the polarization direction of a light beam is controlled by a polarization filter. Also, non-patent document 1 includes a description of an optical element having a structure where two media (for example, air and an isotropic medium) with different refractive indices are arranged alternately at a pitch smaller than the wavelength of light (subwavelength structure: SWS). Such an optical element shows an optical anisotropy called form birefringence.
Conventionally, a birefringent crystal such as rock crystal or calcite has been used to produce birefringence. However, since birefringence is a substance-specific property, it is difficult to control the birefringence of a substance. On the other hand, form birefringence can be produced without using a birefringent crystal and can be relatively easily controlled by changing the shape of a medium.
Using form birefringence makes it possible to create, for example, a polarization beam splitter without using a birefringent crystal. Also, it is possible to form an antireflection structure on an optical surface by changing the shape of a medium and thereby controlling the effective refractive index of the medium.
Form birefringence is also seen in a periodic structure (resonance structure) where two media are arranged alternately at a pitch within a so-called resonance range that is equal to or several times greater than the wavelength of light. An optical element having a subwavelength structure or a resonance structure that shows form birefringence as described above may behave differently with TE-polarized light and TM-polarized light. For example, the diffraction efficiency of such an optical element may become polarization-dependent.
By changing the thickness, an optical element having form birefringence can also be used as a λ/2 plate or a λ/4 plate that changes the phase difference between TE-polarized light and TM-polarized light.
When the refractive indices of TE-polarized light and TM polarized light are n(TE) and n(TM), the wavelength of light is λ, and the thickness of a subwavelength structure is d, the phase difference Φ can be obtained by the following formula:Φ=2Π{n(TE)−n(TM)}d/λ
Also, with a subwavelength structure having a certain thickness d, it is possible to create a polarization filter that transmits only either TE-polarized light or TM polarized light.
[Patent document 1] Japanese Patent Application Publication No. 8-330661
[Patent document 2] Japanese Patent Application Publication No. 9-288244
[Patent document 3] Japanese Patent Application Publication No. 2002-040350
[Patent document 4] Japanese Patent Application Publication No. 2003-215485
[Patent document 5] Japanese Patent Application Publication No. 8-56049
[Patent document 6] Japanese Utility Model No. 2555317
[Patent document 7] Japanese Patent Application Publication No. 10-325933
[Non-patent document 1] Light control by subwavelength grating structure, H. Kikuta and K. Iwata, Japanese Journal of Optics Vol. 27 No. 1 (1998) page 12-17